Nuclear reactor fuel element and method of manufacture



3 2,934,482 Patentet l Apr.v 26, 1,960,.

NUCLEAR REACTGR FUEL ELEMENT AND' METHOD OF MANUFACTURE Harvey Brooks, Cambridge, Mass, assignor to the United States of America as represented by the United States Atomic Energy Commission N0 Drawing. Application September 7, 1950 Serial No. 183,655

2 Claims. (Cl. 204-1932) a which the fissionable material is incorporated, the socalled fuel elements, must be designed so that the heat generated by the fissions and other heat generating effects'may be efficiently transferred to the coolant material passing through the reactor. Ano'ther problem arises from the change in the physical characteristics of fission able material as a result of the fissioning of a portion thereof. These intermediate or fast reactors utilize as the'='=fuel material, plutonium or uranium enriched in respect to the 235 or 233 isotope. It has been-found that these materials when in the'metallic form tend to disintegrate-to a powder after a certain portion of the material has fissioned. It is important, therefore, that the fissionable material be incorporated in a manner that it is not dependent-upon itself for support, for a change in the relative locationof the fissionable maerial Will de1eteriously affect the operating characteristics of the reactor. Still another problem is t0=find a design which will alleviate or mitigate the problem of disposing of gaseous fissio'n products generated during the operation of the reactor.

I An object of this invention is to provide structures adapted to use in a nuclear reactor.

A further object is to provide compositions suitable for use in fuel elements of a nuclear reactor.

A still further object is to provide a fuel element for a nuclear reactor which is relatively resistant to radiation damage. Another object is to provide a fuel element for a nuclear reactor wherein the fissionable material is prevented from undergoing mechanical disintegration. Other objects will'appear hereinafter.

The applicant has found that these several objects are attained by the preparation of compacts wherein the fissionable material is dispersed as finely divided aggregates in a matrix of a dilutant materiah The process of preparation consists in hot pressing the mixed powders of the fissionable material and the dilutant material.

'Broadlyrthe term fissionable material is used in this application to designate metallic uranium or plutonium and their compounds. To sustain a chain reacftionin an intermediate or fast'type of reactor, the uranium "utilized 'is necessarily enriched 'as-to the fissionable isotopes of f U??? or U the degree of enrichment, however, forms nofpart' ofithe present "inventio'n' The compounds of fissionable'material suitable for usein' the practice of this invention are those which do not undergo decomposition at the temperature of hot pressing 'of the compacts.

Uranium dioxide and UAl are examples of 'this type' 'of compound.

The selection of the several criteria. If the dilutant material it guided 6y compact is to be used in an intermediate or fast reactor, thedilutant material-must be one having substantial erities. adsorption cross section the reactor is ly no neutron moderating prop The dilutant must also have a low neutron in the energy region at which to be operated. The mechanical strength of the dilutant must be' relatively good, and it should have a high thermal conductivity.

In connection with the last property, it is also desirable that the dilutant form no intermetallic{compoundsvwith the fissionablev mate rial since these compounds are ceramic-likev and have a very low thermal conductivity. Their-presence would,

, of course, inhibit the tran material to the dilutant. which exhibit the above the dilutant of the subjec sfer of heat from the fissionable Magnesium, calcium and barium properties serve excellently; as tinvention.

It is known that magnesium and calcium do not'alloy' with uranium, does not.

and there is reason to believe that barium Studies of the compacts prepared in the course of this invention indicate that uranium and magnesium do not form intermetallic diffusion found.

compounds, and no evidence of between the uranium and magnesium could be Attempts to form uranium-calcium alloys have proved equally unsuccessfulz" Observations have notindicated any tendency on the-part of plutonium to form alloys or compounds with magnesium or calcium. Since no diffusion or alloying merely as a glue materials. Though, of

takes place, these materials :act

to the particles ofthe fissionable these materials, magnesium is generally to be preferred because of its higher mechanical strength, superior. nuclear properties .and relative chemical inertness, the higher melting points of calcium and:

barium are important considerations where reactor operating temperatures above are desired.

As set forth above, the

the meltingpoint of magnesium.

structures of this invention are prepared by hot-pressing a mixture of the powdered components. The fineness of to be critical. However, the fissionable material distribution is obtained, have the materials in a the powders has not been found to assure that no segregation of occurs,- but that-good random it has been found desirable to fineness of less than 60 mesh,

and preferably of from 200 to 300 mes It has been foundthat the shape of the particlesrof' the fissionable material,

properties of the compact.

preparing some of the- -may affect the heat transfer The uranium powder used in compacts was in the form of platelets which it .Wasfound laid fiat in the transverse direction of the compress.

greater cross-sectional area 0 in a longitudinal directionand decreases the overall heat conductivity in this direction.

This orientation producesa' f poorly conducting uranium particles are used, the orientation of the uranium particles is eliminated and the heat conductivity is uniform in all directions.

The hot-pressing should be carried out at a temperature at whichthe matrix material is practically molten.=. This.

insures that the matrix material flows around the parti-r cles of fissionable material which then exist asv small, islands in the matrix. Such .a structure is desirable since; radiation damage to the fuel element is minimized by the,

isolation of the individual A terial in pockets separated by sound'matrix material I while cold and subsequ The pressure applied has pieces of thegfissionable. ma--;

ently sintered under pressure? not been found to be'extreniely 3 :ritical, however, pressures of from 1500 pounds per ;quare inch and higher assure maximum density, that s, approaching theoretical density, which is generally lesirable.

To protect the components from oxidation during the sintering, the operation should be carried out in an inert atmosphere. A vacuum technique may be used but is less desirable because of the tendency of the matrix materials to distill off at the sintering temperature. Apparatus such as described in C. G. Goetzel, Treatise on Powder Metallurgy, vol. I, pp. 469-495, Interscience Pub., New York (1949), is suitable for the pressing and sintering operations.

The selection of the die material is dependent upon the result desired. If a body solely of the matrix and fissionable material is desired, a die material that will not react or bond to these substances should be chosen. Graphite has been found satisfactory for this purpose.

A unique and valuable result of the bonding properties was noted, however, in a series of pressings of the compacts in stainless steel containers. Because of its corrosion resistance, stainless steel is a useful material for cladding the fuel element to protect the fissionable material from reaction with the coolant passing through the reactor. The selection of any cladding material, however, is dictated not only by its corrosion resisting properties, but also by the type of bond which it forms with the compact of the fuel material. Since iron and uranium form brittle intermetallic compounds, it appeared that it might not be possible to directly press the compacts within the stainless steel containers. It was discovered, however, that when uranium and magnesium mixtures were pressed in steel tubes, although the dispersion of uranium remained homogeneous showing that the uranium did not segregate, as might be predicted from the large density difference, the uranium did not extend to the walls of the steel tube but a thin ring of magnesium about of an inch thick was in contact with the tube. Examination of the bond disclosed it to be purely mechanical and subsequent testing proved that the steel-magnesium bond was strong mechanically, remained firm through repeated thermal cycling, and had an excellent heat transfer coefilcient. It is apparent, therefore, that these properties afford a valuable method for preparing the compacts with a firmly bonded coating of stainless steel. Compacts completely enclosed in stainless steel and firmly bonded to the steel may be prepared by compressing the powder mixture within a stainless steel tube with steel cups placed on the top and bottom of the powder. By hot pressing this setup, a structure will be produced wherein the compact is bonded to the wall of the container and the cups at either end. Complete enclosures may then be obtained by welding the caps to the tubing. Alternatively the pressing may be carried out in a stainless steel tube having a single open end Silg in this instance a cap of stainless steel at the open en Specific examples of the practice of this invention are set forth below.

Example I An 8.8 gram mixture of 20 volume percent uranium powder of minus 60 mesh and 80 volume percent magnesium powder of approximately the same fineness was placed in a I.D. stainless steel tube. Pressure was applied by stainless steel plungers oxidized and coated with Aquadag. The mixture was first cold pressed at 1500 pounds per square inch. After release of this pressure the compact was heated to 650 C. in eleven minutes. A pressure of 2000 pounds per square inch was then applied and the temperature maintained at 650 C. forfifteen minutes. The heating was then stopped, the compact was cooled to 575 C. and the pressure released.

The density of the completed compact was 5.19 grams A. per cubic centimeter. The compact appeared to be firmly bonded to the stainless steel tube and upon pulling the tube off of the compact, part of the outer layer of the compact came off with the tube showing excellent bonding.

Example 11 A mixture of powders containing 42% uranium by volume was pressed under conditions substantially the same as in Example I. A compact of desirable quality was formed although it was observed that the bonding to the stainless steel tubing was somewhat impaired due to the decreased amount of magnesium.

Example 111 Using magnesium powder, a one inch long pressing was made in a inch stainless steel tube using graphite punches with steel cups on the top and bottom of the powder. This mixture was pressed at 650 C. and 2000 pounds per square inch. The tube with the magnesium bonded therein was thermal-cycled between and 500 C. No change was observed in the dimensions of the tube and microexamination of the bond revealed it to be unimpaired. No trace of alloying between the stainless steel and magnesium could be seen.

Continued operation of a neutronic reactor inherently results in the depletion of the fissionable material which was initially incorporated in the reactor. The destruction of the fissionable material is, however, in no manner complete when it becomes profitable to move the fuel elements depleted with respect to the fissionable material contained therein and replace them with new elements. Since fissionable material is only obtained with a high economic cost, it is apparent that it is desirable to recover as much fissionable material as possible that remains in the discarded elements. The elements of this invention afford the use of a simple method of separating the fissionable material and the dilutant. The separation can be obtained by simply distilling off the dilutant material from the relatively non-volatile fissionable material. This procedure eliminates the long, costly and involved chemical separation process.

It is apparent from the foregoing that the present invention provides elements suitable for use in a nuclear reactor and further provides an easy and practical method by which they may be prepared.

I claim:

1. A fuel element for a nuclear reactor comprising a body consisting of from 20% to 42% by volume of finely divided uranium dispersed in a matrix of magnesium metal, a stainless steel jacket enclosing said body and a bonding layer of magnesium interposed between said body and said jacket.

2. The method of forming a fuel element for a nuclear reactor comprising a stainless steel container, a body of finely divided uranium dispersed in a matrix of magnesium within said container and a bonding layer of magnesium metal between said container and said body which comprises placing a mixture of from 20% to 42% by volume of finely divided uranium and finely divided magnesium within a stainless steel container and heating to a temperature between about 630 and 650 C. and simultaneously compressing said mixture in contact with said container.

References Cited in the file of this patent UNITED STATES PATENTS (Other references on following page) it) i 1 5 GT HERj gREFERENCES Treatise on Powder Metallurgy, by Goet z'el (1949),

'vol. 1, pages 436-468; published by Interscience Publishers, Inc., N.Y,

Friend: Textbook of Inorganic Chemistry, vol. VII, part III, page 278 (1926); publ. by Charles Grimm & Co., Ltd., London Chipman: Metallurgy in the Development vof Atomic pages 304 and 305. 

1. A FUEL ELEMENT FOR A NUCLEAR REACTOR COMPRISING A BODY CONSISTING OF FROM 20% TO 42% BY VOLUME OF FINELY DIVIDED URANIUM DISPERSED IN MATRIX OF MAGNESIUM METAL, A STAINLESS STEEL JACKET ENCLOSING SAID BODY AND A BONDING LAYER OF MAGNESIUM INTERPOSED BETWEEN SAID BODY AND SAID JACKET. 